Lithium secondary batteries have features such as high energy density and long service life. Therefore, lithium secondary batteries are widely used as power supplies for electric appliances such as video cameras; portable electronic devices such as laptop computers and mobile telephones; and electric tools such as power tools. Recently, lithium secondary batteries are also applied to large-sized batteries that are mounted in electric vehicles (EV), hybrid electric vehicles (HEV) and the like.
A lithium secondary battery is a secondary battery having a structure in which, at the time of charging, lithium begins to dissolve as ions from the cathode and moves to the anode to be stored therein, and at the time of discharging, lithium ions return from the anode to the cathode, and it is known that the higher energy density of the lithium secondary battery is attributable to the electric potential of the cathode material.
Examples of this kind of cathode active material for lithium secondary batteries include lithium transition metal oxides having lamellar structures, such as LiCoO2, LiNiO2, and LiMnO2; and lithium transition metal oxides having a manganese-based spinel structure (Fd-3m), such as LiMn2O4 and LiNi0.5Mn1.5O4.
Since manganese-based spinel type lithium transition metal oxides of this kind are provided at low prices of raw materials, are non-toxic and safe, and have properties of being resistant to over-charging, attention is paid to them as the next-generation cathode active material for the large-sized batteries of electric vehicles (EV), hybrid electric vehicles (HEV) and the like. Furthermore, since spinel type lithium transition metal oxides (LMO) that are capable of three-dimensional insertion and release of Li ions have superior power output characteristics compared with lithium transition metal oxides having a lamellar structure such as LiCoO2, much expectation is placed on the use of the spinel type lithium transition metal oxides in the applications where excellent power output characteristic are required, such as in batteries for EV and batteries for HEV.
Among others, it has come to be known that when some of the Mn sites in LiMn2O4 are replaced by other transition metals (Cr, Co, Ni, Fe, or Cu), the compound acquires an operating potential close to 5 V. Thus, currently, development of a manganese-based spinel type lithium transition metal oxide having an operating potential of 4.5 V or higher (5 V class) is actively underway.
For example, Patent Document 1 discloses, as a cathode active material for lithium secondary batteries exhibiting an electromotive force of 5 V class, a high capacity spinel type lithium-manganese composite oxide cathode active material containing chromium as an essential additive component to spinel type lithium-manganese composite oxide, and further having nickel or cobalt added thereto.
Patent Document 2 discloses a crystal having a spinel structure, LiMn2-y-zNiyMzO4 (provided that M: at least one selected from the group consisting of Fe, Co, Ti, V, Mg, Zn, Ga, Nb, Mo and Cu; and 0.25≦y≦0.6, and 0≦z≦0.1), which performs charging and discharging at a potential of 4.5 V or higher with respect to Li metal.
Patent Document 3 discloses a spinel type lithium-manganese composite oxide represented by Lia(MxMn2-x-yAy)O4 (wherein 0.4<x, 0<y, x+y<2, 0<a<1.2; M includes one or more kinds of metal elements selected from the group consisting of Ni, Co, Fe, Cr and Cu and contains at least Ni; A includes at least one metal element selected from Si and Ti, provided that when A includes only Ti, the value of the proportion y of A is such that 0.1<y), as a cathode material for lithium ion secondary batteries having a high energy density and having a high voltage of 4.5 V or higher with respect to Li.
Patent Document 4 discloses, as a cathode active material which has a high capacity density by having both the tap density of the cathode active material and the initial discharge capacity of a secondary battery formed by using the cathode active material, a lithium-nickel-manganese composite oxide having a spinel structure represented by formula (I): Li1+xNi0.5-1/4x-1/4yMn1.5-3/4x-3/4yByO4 (wherein in formula (I), x and y are such that 0≦x≦0.025, and 0<y≦0.01), characterized in that the median diameter is 5 μm to 20 μm, the coefficient of variation of particle size is 2.0% to 3.5%, and the BET specific surface area is 0.30 m/g to 1.30 m/g.